The present invention relates to a new and improved method and apparatus for making an object. The method and apparatus can advantageously be utilized to effect the accurate application of a covering having either a uniform or nonuniform thickness to a workpiece, that is to any base material capable of receiving the covering.
There are many known prior art devices for applying a covering to a workpiece. Thus, U.S. Pat. No. 3,953,704 discloses the concept of applying a covering of metal to a workpiece which is a ceramic substrate. The metal covering is applied by a plasma spray gun and the workpiece can be positioned along any one of three axes by a drive apparatus. Although this patent contemplates that the workpiece will be moved along three axes, the patent does not contemplate that the workpiece will be rotated about any of the axes to vary the angular orientation of the workpiece relative to the plasma spray gun.
The concept of rotating a welding nozzle about an axis to apply a bead to the inside of a workpiece which is a pump casing, is disclosed in U.S. Pat. No. 3,627,973. The apparatus disclosed in this patent is utilized to repair the inside of the pump casing by depositing a plurality of beads of material along the inside of the casing. The welding nozzle is movable along X, Y, and Z axes. Although the welding nozzle cannot be rotated about the X and Z axes, the welding nozzle can be rotated about the Y axis. Other devices for applying coverings to workpieces are disclosed in U.S. Pat. No. 3,769,486 and 3,865,525.
The concept of applying a plasma spray coating of from 30 to 40 thousandths of an inch in thickness to a workpiece, that is, a turbine vane, is disclosed in U.S. Pat. No. 4,028,787. The well known process of rebuilding turbine vanes by manually applying a plasma spray covering can, at best, obtain a surface finish accuracy of plus or minus six mils (0.006 inches). The plasma spray build-up must be abraded in order to provide the desired surface measurements.
The application of thermal barrier coverings to airfoils has been attempted on an experimental basis using manually manipulated plasma spray guns. These experiments with the manual application of a plasma spray covering to an airfoil have been less than successful. This is due to the fact that it is a very slow process and the thickness of the plasma spray coating can, at best, be applied with an accuracy of plus or minus 6 mils (0.006 inches). In order to be acceptable for use in a turbine engine, a coating should be applied to the airfoil with an accuracy of at least plus or minus 1.5 mils (0.0015 inches).
The concept of applying coverings to airfoils are also disclosed in U.S. Pat. Nos. 3,068,556 and 1,755,321. These patents contemplate that the coverings will be more or less uniform in thickness. The concept of using plasma spray techniques to deposit a wear resisting covering of a nonuniform thickness to a workpiece is disclosed in U.S. Pat. No. 4,003,115. In addition, the concept of using electroslag welding procedures to apply metal from a plurality of electrodes in layers of different thicknesses to a workpiece is disclosed in U.S. Pat. No. 3,778,580.
The aforementioned prior art patents disclosing different methods of applying coverings to workpieces do not disclose how the thickness of the covering is to be measured. However, there are many known devices for measuring distances. Thus, U.S. Pat. No. 3,940,608 discloses an optical sensing apparatus which is utilized to determine the displacement of an object from a given position. This patent contemplates that the distance will be measured by measuring the intensity of the light reflected from the object and determining the location of that intensity on a curve which compares the intensity of the reflected light to the distance from the object.
The concept of measuring changes in the location of an object by detecting changes in the location of the focal point of a lens is disclosed in U.S. Pat. No. 3,016,464. However, this patent does not disclose the concept of using such a sensing apparatus in association with an apparatus which is utilized to control the application of a covering to a workpiece.